The notification worker holds a reference on the pse_control, but not
on the PHY that psec->attached_phydev points to. If the PHY is removed
while the worker runs, the pse_control survives and the worker reads the
freed PHY. The same happens without a race when two PHYs share one PI:
attached_phydev keeps pointing at the first PHY after it is removed.

pse_control_put() unlinks and frees a pse_control under pse_list_mutex,
and phy_device_remove() calls it before the PHY is freed. So instead of
taking a reference, let the worker look the pse_control up and use
attached_phydev under pse_list_mutex. This nests pse_list_mutex inside
rtnl; nothing under pse_list_mutex takes rtnl.

For a PI shared by two PHYs there is no single netdev to report to, so
clear attached_phydev when a second PHY takes the pse_control, and warn.
Events for that PI are then no longer reported, even after one PHY goes
away. Failing the second PHY instead would fail its whole MDIO bus.

Fixes: fa2f0454174c ("net: pse-pd: Introduce attached_phydev to pse control")
Signed-off-by: Carlo Szelinsky <github@szelinsky.de>
---
 drivers/net/pse-pd/pse_core.c | 42 ++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/drivers/net/pse-pd/pse_core.c b/drivers/net/pse-pd/pse_core.c
index 65a41c517b35..336f00d82b5f 100644
--- a/drivers/net/pse-pd/pse_core.c
+++ b/drivers/net/pse-pd/pse_core.c
@@ -238,24 +238,22 @@ static int of_load_pse_pis(struct pse_controller_dev *pcdev)
  * @pcdev: a pointer to the PSE
  * @id: index of the PSE control
  *
- * Return: pse_control pointer or NULL. The device returned has had a
- *	   reference added and the pointer is safe until the user calls
- *	   pse_control_put() to indicate they have finished with it.
+ * Must be called with pse_list_mutex held. No reference is taken: the
+ * pointer is only valid until pse_list_mutex is released.
+ *
+ * Return: pse_control pointer or NULL.
  */
 static struct pse_control *
 pse_control_find_by_id(struct pse_controller_dev *pcdev, int id)
 {
 	struct pse_control *psec;
 
-	mutex_lock(&pse_list_mutex);
+	lockdep_assert_held(&pse_list_mutex);
+
 	list_for_each_entry(psec, &pcdev->pse_control_head, list) {
-		if (psec->id == id) {
-			kref_get(&psec->refcnt);
-			mutex_unlock(&pse_list_mutex);
+		if (psec->id == id)
 			return psec;
-		}
 	}
-	mutex_unlock(&pse_list_mutex);
 	return NULL;
 }
 
@@ -268,6 +266,7 @@ pse_control_find_by_id(struct pse_controller_dev *pcdev, int id)
 static struct net_device *pse_control_get_netdev(struct pse_control *psec)
 {
 	ASSERT_RTNL();
+	lockdep_assert_held(&pse_list_mutex);
 
 	if (!psec || !psec->attached_phydev)
 		return NULL;
@@ -1020,13 +1019,21 @@ static void pse_send_ntf_worker(struct work_struct *work)
 		struct net_device *netdev;
 		struct pse_control *psec;
 
-		psec = pse_control_find_by_id(pcdev, ntf.id);
+		/* A pse_control is only on the list while the PHY it was
+		 * handed to holds it: the PHY's pse_control_put() unlinks
+		 * and frees it under pse_list_mutex before the PHY itself
+		 * can be freed. Holding the mutex across the lookup and the
+		 * attached_phydev dereference therefore keeps the PHY alive,
+		 * which a reference on the pse_control alone would not.
+		 */
 		rtnl_lock();
+		mutex_lock(&pse_list_mutex);
+		psec = pse_control_find_by_id(pcdev, ntf.id);
 		netdev = pse_control_get_netdev(psec);
 		if (netdev)
 			ethnl_pse_send_ntf(netdev, ntf.notifs);
+		mutex_unlock(&pse_list_mutex);
 		rtnl_unlock();
-		pse_control_put(psec);
 	}
 }
 
@@ -1417,6 +1424,19 @@ pse_control_get_internal(struct pse_controller_dev *pcdev, unsigned int index,
 
 	list_for_each_entry(psec, &pcdev->pse_control_head, list) {
 		if (psec->id == index) {
+			/* attached_phydev can only name one PHY. If a second
+			 * one shares the PI, either may be removed first and
+			 * leave it pointing at a freed phy_device, so stop
+			 * reporting events for this PI instead.
+			 */
+			if (psec->attached_phydev &&
+			    psec->attached_phydev != phydev) {
+				dev_warn(pcdev->dev,
+					 "PI %u shared by several PHYs, not reporting its events\n",
+					 index);
+				psec->attached_phydev = NULL;
+			}
+
 			kref_get(&psec->refcnt);
 			return psec;
 		}
-- 
2.43.0